Tomasz Majka – V rokKoło Naukowe Chemikówdr inż. Agnieszka Leszczyńska – opiekun naukowy

The synthesis and investigation of structure – properties relationship inpolyoxymethylene (POM) / montmorillonite (MMT) nanocomposites.

Otrzymywanie i badanie zależności struktura – właściwości nanokompozytówpolioksymetylen (POM) / montmorylonit (MMT).

Keywords: polyoxymethylene, montmorillonite, nanocomposites, mechanical properties, thermo-oxidative degradation,Słowa kluczowe: polioksymetylen, montmorylonit, nanokompozyty polimerowe, właściwościmechaniczne, degradacja termooksydacyjna,

Abstract

Polymer composites are important commercial materials with good mechanical and thermalproperties dependent on interface interaction and aspect ratio of fillers. In recent years, polymer-nanoparticle composite materials have attracted the interest of a number of researchers, due to theirsynergistic and hybrid properties derived from several components. Nanocomposites are a newclass of composites with small contents of nanoscale size fillers and excellent properties. In thiswork the results of kinetic analysis of thermooxidative degradation of POM/MMT nanocompositesmodified with ammonium and imidazolium salts were presented. The influence of onium salt onkinetic parameters of degradation process was studied. Simulations of isothermal decompositionsin oxidative atmosphere allowed to compare the rate of nanocomposites’ decomposition in 265˚C.The changes in thermal properties were explained in terms of limited oxygen diffusion intomaterial. In case of the imidazolium salt the deterioration of thermal stability can be bounded withextensive production of volatile compounds via autocatalytic process produced by degradationproducts of organomodifier. The heat distortion temperature (HDT) and hardness ofnanocomposites were found to increase as compared with neat POM, regardless of the type ofonium salt applied for MMT modification. The WAXD diffractograms confirmed the intercalationand/or exfoliation of aluminum silicate layers dispersed in POM matrix. Nanocomposites ofpolyoxymethylene (POM) and organo-modified montmorillonite containing quaternary aminesexhibited improved tensile strength and modulus as well as increased elongation at break. Bothmechanical and thermal properties of nanocomposites varied depending on type of ammoniumsurfactant used for organomodification of MMT. The addition of imidazolium salt caused somedetrimental effect on the mechanical properties of these systems.

Kompozyty polimerowe są ważnymi komercyjnymi materiałami o polepszonychwłaściwościach mechanicznych i cieplnych, zależnych od oddziaływań międzyfazowych orazwspółczynnika kształtu napełniaczy. W ostatnich latach materiały kompozytowe na osnowiepolimeru modyfikowane przez dodatek nanocząstek przyciągnęły zainteresowanie dużej liczbyośrodków badawczych z powodu ich hybrydowych właściwości i często obserwowanego efektusynergistycznego. Dodatkowo poprawę właściwości w nanokompozytach polimerowych uzyskuje

się przy małych zawartościach napełniaczy. Dla uzyskanych nanokompozytów POM/MMTobserwowano wzrost twardości i temperatury mięknienia w porównaniu do naturalnego POM.Gęstość nanokompozytów nie ulegała zasadniczej zmianie, jak w przypadku klasycznychkompozytów napełnianych np. włóknem szklanym. Wykonane badania metodą szerokokątowejdyfrakcji rentgenowskiej pokazały zanik regularnej struktury warstw glinokrzemianu wnanokompozytach POM z dodatkiem soli imidazoliowej wskazujący na dobrą homogenizacjęnanododatku. W materiale zawierającym handlowy gatunek OMMT (Nanofil N3010) doszło dointerkalacji polimeru pomiedzy warstwy MMT i zachowanie uporządkowanej struktury warstwglinokrzemianu. Przeprowadzono pomiary wytrzymałości na rozciąganie nanokompozytów zdodatkiem soli amoniowej oraz imidazoliowej. Nanokompozyty polioksymetylenu (POM) imontmorylonitu (MMT) organofilizowanego za pomocą związków powierzchniowo czynnychtypu amin czwartorzędowych, wykazują zwiększoną wytrzymałość mechaniczną.Zaobserwowano, iż dodatek soli imidazoliowej niekorzystnie wpływa na niektóre właściwościmechaniczne tych systemów. Poprawa właściwości termicznych nanokompozytów w warunkachutleniających związana była z efektem barierowym, polegającym na ograniczeniu dyfuzji tlenu downętrza próbki przez zdyspergowane warstwy MMT. Pogorszenie stabilności termicznej wprzypadku zastosowania soli imidazoliowej może być związane z efektem autokatalitycznymwywołanym obecnością produktów degradacji organicznego modyfikatora montmorylonituwywołującego acydolizę polioksymetylenu. Zaprezentowano wyniki analizy kinetycznejdegradacji nanokompozytów POM/OMMT modyfikowanych dodatkiem soli amoniowej iimidazoliowej w atmosferze powietrza. Następnie z wykorzystaniem metody regresji nielinowejposzukiwano teoretyczny model reakcji o najlepszym dopasowaniu do wynikóweksperymentalnych w pierwszym etapie degradacji.

1. Introduction

Polymer nanocomposites are materials composed of two phases – organic polymer(continuous phase) and inorganic filler (dispersed phase), obtained mostly as a result of themodification of traditional polymeric materials by dispersing nanoparticulate fillers. As a polymermatrix both thermoplastic and thermosetting polymers can be used. Depending on the properties ofpolymer and layered silicates three types of nanocomposites can be obtained (Fig. 1):

® conventional nanocomposites,® intercalated nanocomposites,® exfoliated nanocomposites.

Unfortunately, the nanoscale dispersion of filler in polymer matrix is usually very difficult toachieve, due to the high surface activity of a nanofillers and its tendency for agglomeration. Inorder to improve the dispersion of nanofillers different treatments of filler are applied. Mostpopular method of surface modification of MMT is cation exchange reaction, where inorganiccations from interlayer space are displaced by organic ones, eg. ammonium or phosphoniumcations.

There are four principal methods for producing polymer – layered silicate nanocomposites:® intercalation of polymer or prepolymer from solution,® in situ intercalative polymerization,® melt intercalation,® template synthesis (sol-gel technology).

Figure 1. Schematically illustration of three different types of thermodynamically achievable polymer/layeredsilicate nanocomposites [1,2].

2. Tasks

The aims of this study were:

® Melt mixing of polyoxymethylene (POM) with montmorillonite (MMT) using laboratorytwin-screw extruder;

® Study on the effects of nanofillers on physical (hardness and density), mechanical (tensilestrength) and thermal (the heat distortion temperature and thermal stability) properties ofPOM / MMT nanocomposites;

® Determination of the effect of nanofillers on the kinetic parameters and mechanism ofthermal oxidative degradation of polyoxymethylene and its compositions;

® Investigation of the effect of ammonium and imidazolium salt on the properties of POMnanocomposites.

3. Experimental part

Preparation of the material and test samples:® Preparation of POM / MMT nanocomposites by homogenization in melt using a twin

screw extruder - HAAKE Reomex OS PTW 16 with screw diameter 16 mm and L/D ratio25 (Fig. 2a). The processing parameters were: temperatures of the barrel zones - 175, 190,190, 200 i 200ºC, the screws speed – 100 rpm. The selected weight contents of nanofillerswere used: 1, 3 and 5%.

® Obtaining injection bars of POM / MMT nanocomposites in the laboratory pneumaticinjection molding machine - ZAMAK WT-12 ZMK/102/09 (Fig. 2b) at the temperaturesof barrel and mould 200ºC and 80ºC respectively.

Figure 2. Illustration of: a) a twin screw extruder - HAAKE Reomex OS PTW 16; b) laboratory pneumaticinjection molding machine - ZAMAK WT-12 ZMK/102/09.

MethodsX-ray diffraction experiments were carried out on a Philips X-Pert diffractometer, with graphitemonochromator placed in the front of detector - lCu = 1.5418 Å. Thermogravimetric analysis wasperformed on a Netzsch TG 209 thermal analyser, operating in a dynamic mode at a heating rate of10 K/min. The conditions were: sample weight - ~ 5 mg, atmosphere - argon, open a-Al2O3 pan.The heat distortion temperature was measured according to Vicat method.

4. ResultsThe Wide X-ray diffraction studies performed for the nanocomposites containing

montmorillonite with ammonium compounds showed changes in the structure of layered silicatesdue to intercalation of polymer in the gallery spaces occurring in the extruder. The imidazoliumcontaining nanofillers underwent complete exfoliation in POM since the diffraction peakscorresponding to the regular structure of MMT was not recorded on the WAXD spectra ofPOM/ImidS nanocomposites. (Figs. 3a, 3b).

An increased heat distortion temperature was observed for POM modified by both types ofnanofiller - with ammonium and imidazolium salts (Table 1). For compositions of POM/NanofilN3010 temperature increased for about 3.3˚C with respect to the pure polyoxymethylene, and forthe composition of POM/ImidS the increase was about 7.2˚C. This results can be explained interms of the improved mechanical properties at high temperatures due to strengthening of polymerby highly anisometric nanoparticles. The restriction of thermal movement of individual moleculesand whole polymer segments due to interfacial interactions should also be considered as aprobable phenomena operating in raised temperatures. Higher effect was observed in imidazolium-containing nanocomposites probably due to higher degree of MMT dispersion (exfoliation) anddeveloped interfacial sufrace.

a) b)

Figure 3. Illustration of: a) WAXD diffractogram Nanofil N3010 and montmorillonite modified witha imidazolium salt; b) WAXD diffraction pattern of nanocomposite POM/Imids and POM/Nanofil N3010.

Table 1Test results of the heat distortion temperature measurement according toVicat method.

Sample Heat Distortion Temperature [˚C]

POM 155,0POM/Nanofil N3010 3% 158,3

POM/ImidS 3% 162,2

The results of hardness tests show that even a small addition nanofillers increases hardness ofthe material. The change of hardness depended on the type of salt used for the filler modifficationand the amount of filler. The best results were obtained for the filler modified by imidazolium salt.The addition of 3wt% of OMMT into the POM increased its hardness by approximately 10.44%(Table 2).

Table 2 Hardness of POM and POM/MMT nanocomposites.

Sample Hardness [MPa] Increase in hardness [%]POM T200 97,7 -

POM/Nanofil N3010 1% 98,1 0,41POM/Nanofil N3010 3% 98,5 0,82POM/Nanofil N3010 5% 102,9 5,32

POM/ImidS 3% 107,9 10,44

The addition Nanofil did not cause significant changes in the density of the material, whilethe density of the nanocomposite modified with imidazolium salt increased slightly. This effect isadvantageous in terms of potential application of POM/MMT nanocomposites in automotiveindustry. It is worth to mention that conventional composites have significantly increased density,

b)a)

as compared to neat polymers, which contribute to increase of the total weight of final product. Inautomotive industry this leads to higher fuel consumption. Nanocomposites, on the other site, offerimproved mechanical and thermal properties maintaining low specific weight.

The kinetic analysis of the degradation process of all tested nanocomposites was conductedacording to Friedman method. It has shown the increase of the activation energy of nanocompositedegradation at small values of degree of conversion with respect to neat polymer (Figure 4a, 4b,4c). Moreover, the isoconversional lines of nanocomposites had higher slope than the experimentalpoints, which suggested the contribution of autocatalytic reactions in the degradation process.

Figure 4. The results of kinetic analysis of the degradation process in air atmosphere using Friedman method:a) POM; b) POM/OMMT nanocomposite modified with ammonium salt; c) POM/OMMT nanocompositesmodified with imidazolium salt.

Interestingly nanocomposite sample modified with imidazolium salt had a much lower valueof activation energy in the initial stage of thermooxidative degradation process as compared withpure POM. This explains the deterioration on thermal stability of POM/ImidS nanocompositespossibly due to initiation of decomposition by imidazolium compound or products of itsdecompositions at lower temperatures (Fig. 5a, 5b, 5c).

A successive modeling of degradation process in the beginning stage was carried out usingthe nonlinear regression method and kinetic parameters calculated in the Friedman analysis. The

a)

b) c)

b)c)

mathematical model of degradation reactions with the best fit to the series of experimental pointsof all tested samples was in the form of two-stage process: the main reaction and the subsequentreaction (Figure 6). Interestingly, the introduction of naofiller caused change in the geometry ofdiffusion field from 3 dimensional to 1 dimensional. This may reflect the restricted path fordiffusion of low molecular weight products of degradation entrapped between the MMT layers andconfirm the barrier mechanism of polymer stabilization by nanofillers.

Figure 5. The dependence of apparent activation energy (E) and the exponential factor (A), the relative weightloss determined by Friedman to the process of thermal oxidative degradation of: a) Polyoxymethylene; b)POM/OMMT nonocomposite modified with ammonium salt; c) POM/OMMT nanocomposites modified withimidazolium salt.

a)

Figure 6. Results of non-linear regression analysis for the best fitting model of main reactions and subsequentreactions: a) Cn-D3 for sample POM; b) Cn-D1 for sample POM/OMMT modified with ammonium salt;c) Cn-Bna for sample POM/OMMT modified with imidazolium salt.

4. Conclusions

The WAXD studies have shown that the degree of dispersion depends on the type of organicmodifier used for MMT modification. The ammonium surfactant rendered the regular structure ofMMT layers in polymer while imidazolium surfactant enabled complete exfoliation of layeredsilicates. An increase in the melting temperature and hardness was observed due to addition offiller. Similar changes were achieved for nanocomposites containing the ammonium andimidazolium salt. Addition of Nanofil did not cause significant change in material’s density,whereas the use of imidazolium modified clays cased slight increase in density. The kineticanalysis indicated that the mechanism of thermal oxidative degradation process in the first stage ofmass loss of all tested samples were two-stage process – modeling for two consecutive reactionsgave good correlation with the experimental data. The n-order reaction with autocatalytic effect

c)

a) b)

and diffusion controlled reaction were found to rule the degradation of nanocomposite materials.Research on the tensile strength under static deformation of all tested nanocomposites revealedthat these materials are capable of handling the larger stresses.

5. Literature

[1] Pavlidou S.; Papaspyrides C.D.; A review on polymer–layered silicate nanocomposites;Progress in Polymer Science, (2008); Nr. 33, str. 1119 – 1198

[2] Ray S. S.; Okamoto M.; Polymer/layered silicate nanocomposites: a review from preparationto processing; Progress in Polymer Science, (2003); Nr. 28, str. 1539 – 1641

[3] Kacperski M.; Nanokompozyty polimerowe; Kompozyty, (2003); Nr. 7, str. 225 – 232[4] Crosby A.J.; Lee J.Y.; Polymer Nanocomposites: The “Nano” Effect on Mechanical

Properties; Polymer Reviews, (2007); Nr. 47, str. 217–229[5] Stodolak E.; Zych Ł.; Łącz A.; Kluczewski W.; Modyfikowany montmorylonit jako

nanowypełniacz w nanokompozytach polimerowo-ceramicznych; Kompozyty, (2009);Nr. 2, str. 122 – 127

[6] Królikowski W.; Rosłaniec Z.; Nanokompozyty polimerowe; Kompozyty, (2004); str, 3 – 16[7] Sperling L.H.; Introduction to physical polimer science; Willey Interscience, Bethlehem

Pennsylvania, 2006; str. 723 – 736[8] Singla P.; Organic modification of Clay; School of Chemistry and Biochemistry Thapar

University, Thapar, 2009[9] Silvera Batista C.A.; Archer L.A.; Polymer / Silica Nanocomposites; Department of Chemical

Engineering, New York, 2005[10] Xu W.;Ge M.; He P.; Journal of Applied Polymer Science; (2001); 2281, str. 82[11] Pagacz J.; Pielichowski K. Otrzymywanie i ocena właściwości nanokompozytów PVC/MMT

wytwarzanych metodą rozpuszczalnikową; Czasopismo Techniczne, (2009); zeszyt 4,str. 95 – 110

[12] Sestak J.; Thermophysical Properties of Solids. Their Measurements and Theoretical ThermalAnalysis; Elsevier, Amsterdam, 1984